1. Field of the Invention
The present invention relates to an image forming apparatus that performs image formation in a plurality of colors, and is capable of reducing recovery printing time.
2. Description of the Related Art
Conventionally, as a color image forming apparatus using an electrophotographic process, there has been known one using a method of primarily transferring a toner image formed on a photosensitive member onto an intermediate transfer belt and thereafter secondarily transferring the toner image on the intermediate transfer belt onto a recording sheet.
For example, in a tandem-type color image forming apparatus, four photosensitive members for yellow toner, magenta toner, cyan toner, and black toner are arranged along a direction of rotating the intermediate transfer belt. Each of the photosensitive members and the intermediate transfer belt are rotated while being in contact with each other, whereby toner images formed on the respective photosensitive members are sequentially transferred onto the intermediate transfer belt in a superimposed manner to form a color image on the intermediate transfer belt. Then, the color image formed on the intermediate transfer belt is transferred onto a recording sheet by a transfer section located downstream in the rotational direction.
Image formation modes of the apparatus of this type include a color image formation mode for forming a four-color image by driving all of the four photosensitive members, and a monochrome image formation mode for forming an image by driving only the photosensitive member for black toner.
Although a color image is formed by superimposing four color toner images of yellow, magenta, cyan, and black, a monochrome image can be formed using only black toner. That is, toner of three colors of yellow, magenta, and cyan is not required for monochrome image formation, and hence the intermediate transfer belt is not required to be brought into contact with the photosensitive members for toner of the three colors. Therefore, usually, the intermediate transfer belt is brought into contact with all of the four photosensitive members in the color image formation mode, and is brought into contact only with the photosensitive member for black toner in the monochrome image formation mode.
However, it is possible to print a monochrome page by setting the position of the intermediate transfer belt relative to the photosensitive member of each color to a position for color image formation. For example, in continuous printing in which color pages and monochrome pages are printed in a mixed manner, if both of color and monochrome images are formed by bringing the intermediate transfer belt into contact with the four photosensitive members, it is possible to obtain an advantage that there is no need to switch the image formation mode page by page, which reduces the whole printing time by a time period required to switch the mode. However, the above-mentioned printing method causes the photosensitive members for color toner to rotate in the state brought into contact with the intermediate transfer belt even in monochrome image formation, which is not preferable in terms of unnecessarily accelerated abrasion and deterioration of the photosensitive members for color toner.
In view of the above-described circumstances, there has been proposed a method of determining an image formation mode, which aims to satisfy the two requirements of reduction of the whole printing time and prevention of the photosensitive members from being wastefully abraded and deteriorated (Japanese Patent No. 3848177). In Japanese Patent No. 3848177, part of monochrome pages is printed in the color image formation mode depending on how color pages and monochrome pages are mixed. This reduces the number of times of switching between the image formation modes to thereby reduce the whole printing time, and also reduces unnecessarily accelerated abrasion and deterioration of the three photosensitive members for color toner.
However, in Japanese Patent No. 3848177, in a case where a print job is interrupted e.g. due to a paper jam (hereinafter also simply referred to as a jam), even in recovery printing for printing the remaining pages after clearing the jam, the image formation mode is determined by using the same method as that used before the print job is interrupted.
Therefore, the number of times of switching between the image formation modes in recovery printing after interruption of the print job is the same as that in printing the remaining pages in a case where the job is not interrupted. That is, in both of the cases, the same time period is required for the total mode switching time period, and hence the time lost by job interruption e.g. due to a jam is directly added to the total printing time.
In general, when performing a printing operation using an image forming apparatus, a user empirically estimates a time period required to complete the printing operation to some degree of accuracy from the number of pages, how color pages and monochrome pages are mixed, and so on, based on productivity (the printable number of sheets per minute) of the apparatus used for the printing operation. The time period estimated by the user at this time is normally based on the premise that the printing operation is not interrupted in the middle of printing. For example, in a case where documents are required to be printed for the purpose of usage in a meeting, and a time period required for printing is estimated to be 30 minutes, the user starts printing in view of more than half an hour before the start of the meeting.
However, according to the method of determining the image formation mode disclosed in Japanese Patent No. 3848177, if a job is interrupted, a time period required to clear a jam is added to the total printing time period, and hence completion of printing is largely delayed from the finish time estimated by the user. As a result, the print output may be late for the scheduled meeting.
As described above, the apparatus disclosed in Japanese Patent No. 3848177 uses the same method of determining the image formation mode in recovery printing as well. This brings about a problem that it is impossible to properly reduce the recovery printing time depending on the situation, and it is impossible to recover the time lost by job interruption.